1. Field of the Invention
The present invention relates generally to communication networks, and in particular to a wireless station of a wireless data network finding at L2 one or more access points that are in communication with a mobility agent, such that the wireless station can associate with such an access point to enable Mobile IP services.
2. Background
Wireless networks such as wireless local area networks (WLANs) are known. In a WLAN, stations are able to communicate wirelessly within a local network. In an infrastructure network, all communication is via an access point that acts as a base station. Any 3entity on the WLAN can also communicate with a node on any other wired or wireless network that is connected to the WLAN, e.g., via the access point.
Mobile IP is a well known extension of the Internet Protocol (IP) that enables a node on an IP-based network to change its physical attachment point in the network, e.g., the Internet, while retaining the same IP address by which it is identified on its home network. Agents providing Mobile IP services send IP packets that include advertisements of the services they provide. Thus, determining IP services is a process that occurs at L3 of the communication protocol.
While WLANs typically enable a wireless node to move from place to place within a small area, e.g., at a LAN level, without disrupting communications, mobile IP provides IP address mobility across a global network such as the Internet.
A station in a WLAN is not guaranteed to function seamlessly as Mobile IP node. In particular, a prior art wireless station associates with an access point at L2. Because the availability of IP services is determined at L3, prior art wireless stations do not function seamlessly with Mobile IP.
It is desired for a station on a WLAN also to be able to communicate using Mobile IP. In particular, it is desired for the wireless station to determine what IP services, if any, are available to access points prior to attaching to an access point.
The following discussion provides a brief overview of Mobile IP and wireless LANs, including definitions. A review of the need for the invention follows the discussion.
Mobile IP
The purpose of Mobile IP is to enable a mobile node to retain its home IP address even when it is away from its home subnetwork of an overall network such as the Internet. This is accomplished by the addition of a mobility agent, a router known in MIPv4 as a foreign agent, that provides the node with a “care-of” address for the mobile node. MIPv6 provides a slightly different mechanism. The care-of address is part of the IP packet itself, so that foreign agents are not required. The mobile node's home network similarly adds a mobility agent, in this case a router known as a home agent, which forwards IP packets addresses to the mobile node's IP address to the node's care-of address on the foreign network.
When a MIPv4 mobile node determines that it is not attached to its home network, it listens for router advertisements from a foreign agent. Such foreign agent advertisements contain one or more care-of addresses. After obtaining a care-of address, the mobile node then registers with the foreign agent, and subsequently, registers the care-of address with its home agent. Once these registrations are complete, the home agent intercepts IP packets containing the IP address of the mobile node and forwards them, via an IP tunnel, to the care-of address. When these IP packets reach the foreign network, the foreign agent redirects them to the mobile IP node at its care-of address. Thus the foreign agent removes them from the tunnel and the home agent together act as proxies for the mobile node's IP address, routing inbound and outbound traffic to and from the mobile node using its home IP address. When a mobile note again changes its attachment point to the Internet, it locates a new foreign agent, obtains a new care-of address, and re-registers with its home agent.
Note that the providing of Mobile IP services by foreign agents and home agents uses IP packets, and thus occurs at the network layer (layer-3).
According to RFC 3344, there is only one active care-of address for each foreign agent or access router, and therefore a single reverse tunnel to the Home Agent needs to be set up.
Wireless LANs
WLANs conform to one or more known protocols, such as the various versions of the IEEE 802.11 standard. An infrastructure WLAN includes an access point (AP) to act as a base station in the WLAN so that any wireless communication between stations in the WLAN occurs via the AP. Typically, the AP is connected to a wired network such as a LAN which in turn may be connected to a wide area network (WAN) such as the Internet. Any station in the WLAN can thus communicate with both the wired LAN or WAN via the AP.
According to the IEEE 802.11 standards, a station attempts to associate with an AP by scanning in “RF monitor mode” for signals, known as “beacon signals,” that periodically originate from an AP. Beacon signals, in addition to other functions, serve to uniquely identify the AP within the WLAN. When a station detects a beacon, it switches into send-receive mode and executes an association protocol with the AP. At the successful end of the association protocol, the station is said to be associated with the AP.
Once a station associates with an AP, the AP is responsible for delivering network traffic to the station (and other associated stations). While an AP may associate with multiple stations, a station may associate with only one AP at a time, even though there may be more than one AP that the station can physically communicate with. When an associated station loses communication with its AP, the station must re-associate, either with a different AP, or the same AP.
The association between a station and an AP is initiated by a station, not an AP. After receiving one or more beacon signals, the station selects an AP it can communicate with, e.g., after receiving a beacon signal, and begins the association protocol. The criteria for a station selecting one AP over another is not specified in the IEEE 802.11 standard, but in practice, at minimum it is based on the quality of the wireless signal from the available APs. As previously noted, once associated, the station does not directly wirelessly communicate with any station not associated with its AP.
Note that the associating of a station with an AP occurs at the link layer (layer-2) of the protocol. In contrast, the providing of Mobile IP services by foreign agents and home agents uses IP packets, and thus occurs at the network layer (layer-3). Because the association procedure occurs at a lower protocol layer than Mobile IP services, the association between an AP and a station occurs at a protocol layer wherein there is ignorance of Mobile IP services, and thus occurs without regard to whether the AP is in communication with any mobility agents.
Therefore, there is a need in the art for a method and apparatus that enables a station of a WLAN to discover, prior to association, whether one or more candidate APs for association that are in communication with Mobile IP agents, e.g. with home agents and/or foreign agents. There also is a need subsequently to select for association one of the APs in communication with a mobility agent so that the wireless station can benefit from and/or provide Mobile IP services.